1. Field of the Invention
The present invention relates to an air velocity control unit and an air conditioning system including the air velocity control unit. More particularly, the present invention relates to an air velocity control unit for controlling the velocity of clean air released from an air filter disposed in a clean room, and an air conditioning system including the air velocity control unit.
2. Description of the Related Art
Generally, electronic parts including semiconductor devices are fabricated in a clean room that may be maintained at a constant temperature, humidity and pressure. The clean room may be controlled on a clean level with limited floating particles in air.
The clean room includes various kinds of air filters disposed in an air circulation chamber. The air filters remove airborne particles or an airborne molecular contamination (AMC) contained in the air, thereby creating a filtered laminar flow of air. The contaminant level in the clean room is controlled to a great extent by the cleanliness of the laminar air flow.
A bay of the clean room is divided into a process area and a working area. The process area and the working area have different cleanliness levels. The amount of the air-flows handled by the air filter and the filtering efficiency of the air filter are controlled according to the different cleanliness level in the respective process and working areas so that the final contaminant level of the clean room is determined.
Referring to FIG. 1A, a clean room of a process line is divided into a filter zone 15 including a process area 10 and a working area 12, and service zone 14. Each of the respective areas of the filter zone 15 is provided according to a predetermined cleanliness level of the clean room and an incoming air velocity. The clean room is divided into a low clean area and a high clean area. This is accomplished by providing air filters having different filtering efficiencies and air velocities to the respective areas. This separation into respective low and high clean areas facilitates the suppression of the diffusion of contaminants created by the particles in the air flow emitted into the clean room.
For example, an air filter, which removes about 99.9999% of particles having a size of about 0.1 μm and releases air having a velocity of above 0.4 m/s, may be provided to the process area 10 that requires high clean level, and which is represented as a class 1 location. An air filter, which removes about 99.9999% of particles having a size of about 0.1 μm and releases air having a velocity of above 0.25 m/s, may be provided to the working area 12 that requires a intermediate clean level, and which is represented as a class 10 location. An air filter, which removes about 99.9999% of particles having a size of about 0.1 μm and releases air having a velocity of above 0.35 m/s, is provided to the service zone 14 that requires a low clean level, and which is represented as a class 1,000 location.
The filtering efficiency of the air filter is related to maintaining the clean level of the process line in which semiconductor devices are fabricated. The velocity of air released from the air filter is controlled so that particle restricted zones are formed in the respective areas.
However, as time passes, it may become necessary for the filter zone 15 to be changed because of the introduction of new fabrication equipment. As shown in FIG. 1B, this results in the formation of a new area 16. An air filter is additionally installed for use within the area 16. Also, the number of air filters provided to the area 16 is often a substantial number. In order to install additional air filters, or to change the air filter that has been installed, the fabricating process or the operation of the clean room has to be suspended. As a result, this suspension of the fabrication process brings about significant stoppage of research, development and production of the semiconductor devices resulting in the significant loss of the time and money. Also, there is an additional cost due to continuous replacement of the air filters in the clean room.
Furthermore, the change in the clean room environment due to the enlargement of the clean room areas may be difficult to control, and the velocity of the air being released and the temperature in local zones may be hard to control as well. Thus, the level of cleanliness will in turn decrease. For example, when the air filter rapidly releasing air is installed as part of the new equipment, clean air passing through the air filter will collide with the new equipment and turbulent air will be formed. This turbulent air disturbs the laminar flow of downward air that will decrease the cleanliness level in the area.
A typical air conditioning system in a clean room controls the releasing velocity of air by manually operating the flow of the air entering into the clean room. The air conditioning system includes a first flat plate having an opening therein, a second flat plate translationally moving relative to the first flat plate and having an opening therein, and a third flat plate interposed between the first and second flat plates to restrict air flow between the first and second flat plates, respectively. The openings are selectively opened or closed by moving the second flat plate relative to the first flat plate so that the velocity of air entering into the clean room can be controlled.
However, since the area of the opening is limited to less than 30% of the total area of the first or second plate, the pressure loss of air may increase. Furthermore, since the friction resistance of air increases due to vibrations of the plates, particles may be generated from abrasion of the plates.